I. Field of the Invention
The present invention relates to a package structure, and more particularly, to a package structure using lead frames.
II. Description of the Prior Art
Along with the rapid progress of the computer and internet communication during the recent years, the switching power supply is widely applied in the information and communication equipments including personal computers, servers, and routers, etc. . . . . It is well known that a pair of MOSFET is common used as the voltage regulator in the switching power supply.
Please refer to FIG. 1, which is a schematic circuit diagram to demonstrate the operation of the MOSFET pair. It contains two MOSFETs, a first MOSFET 10 and a second MOSFET 20, as shown in FIG. 1, the drain of the first MOSFET 10 and the source of the second MOSFET 20 are electrically connected. The first MOSFET 10 and the second MOSFET 20 will regulate the input voltage from the voltage source Vin, and output to an output capacitor Cout and a device 30, such as a microprocessor through a coupling inductor Lout. Because the switching power supply has developed towards to the high-frequency range presently and there is much parasitic inductance in the circuit of FIG. 1, the Electromagnetic Interference (EMI) has become a serious problem needed to be overcome. Conventionally, there are several techniques has been adopted to tackle the EMI problem. A simple way is utilizing the package technique, for example, the technique of wire-free bonding to reduce the parasitic inductance. However, its improvement is very limited.
Please continuously refer to FIG. 1, wherein another conventional technique is connecting an input capacitor Cin to decouple the EMI. However, additional parasitic inductance will be produced if the input capacitor Cin is mounted on a Printed Circuit Board (PCB). Therefore, it is not an optimized way to minimize the EMI loop.
Another conventional technique to solve the EMI problem is integrating the input capacitor Cin with the MOSFET pair to form a module. Please refer to FIG. 2, which is a cross-sectional schematic diagram to demonstrate the structure of a conventional MOSFET module 100. The module 100 has a pair of MOSFET, a first MOSFET 40 and a second MOSFET 50, which are mounted side by side between a bottom lead frame 60 and a copper plate 70. And, the first MOSFET 40 and the second MOSFET 50 are electrically connected with the bottom lead frame 60 and the copper plate 70 respectively. An input capacitor 80 is mounted on the copper plate 70, such that it is electrically connected with the first MOSFET 40 and the second MOSFET 50 respectively via the copper plate 70. Accordingly, the module 100 can form a short EMI loop; therefore it is more effective to suppress the EMI comparing to other techniques discussed previously.
Nevertheless, the structure of the module 100 has several disadvantages which include: a. The connecting point of the copper plate 70 and the bottom lead frame 60 will induce an impedance to degrade the module performance; and b. The thermal-dissipation ability of the module 100 is limited because it can just dissipate the heat through the bottom lead frame 60 when mounting on a PCB.
Consequently, the present invention proposes a module, which has a pair of MOSFET and an input capacitor stacked on an up lead frame, to overcome the above mentioned problems and disadvantages.